Buffer system for a wafer handling system

ABSTRACT

A processing unit for processing at least one semiconductor wafer includes a processing station for processing the wafer, a measuring station for measuring the at least one water, a robot for moving the wafer between the processing and measuring stations, a wafer handling system and a buffer station. The wafer handling system operates in conjunction with the measuring station and moves the wafer to and from a measuring location on the measuring unit. The buffer station is associated with the wafer handling system and receives measured and unmeasured wafers thereby to enable the robot to arrive at and leave the measuring station with at least one wafer thereon.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.09/097,298, filed Jun. 12, 1998 and assigned to the common assignees ofthe present invention.

FIELD OF THE INVENTION

The present invention relates to handling and robotics systems, ingeneral, and to such in semiconductor processing control systems, inparticular.

BACKGROUND OF THE INVENTION

Reference is made to FIG. 1 which illustrates a prior art processenvironment 10 in a semiconductor fabrication plant. In general, processenvironment 10 comprises a processing unit 2, such as a chemicalmechanical polisher, at least one load/unload cassette station 4 (twoare shown), an integrated metrology tool 6 and a robot 8.

The robot 8 transfers wafers to and from both the processing unit 2 andthe cassette stations 4. However, the integrated metrology tool 6requires its own handling system in order to transfer the wafer to bemeasured from the robot 8 to a measuring location on tool 6 and viceversa.

FIGS. 2A, 2B, 2C, 2D, 2E and 2F illustrate the operation of tool 6 androbot 8 using a handling system 16 to work with an integrated metrologytool having a measuring unit 15. One exemplary process environment usesthe NovaScan 210 integrated metrology tool, commercially available fromNova Measuring Instruments Ltd. of Rehovot, Israel, and its handlingsystem. The handling system 16 is composed of a bent arm 17 connected toa gripper 18. The latter can be any gripper which can hold a wafer. Forexample, it can be a vacuum gripper.

The arm 17 slides vertically on a vertical rail 14 and reaches above themeasuring unit 15 in order to place a new wafer in a measuring positionand/or to return a measured wafer to the robot 8. Between the uppermostposition of gripper 18 and measuring unit 15 there is a supportingstation 19 comprised of two supporting beams 24 and 25, each of whichhas a supporting base 26. Supporting beams 24 and 25 are connected to arail 30 by a relative motion unit 32. Unit 32 is designed to providerelative motion to supporting beams 24 and 25 such that they move towardand away from each other, as indicated by arrows 34 and 36. Supportingstation 19 is connected to the measuring unit 15 by a solid connector54.

As shown in FIG. 2B, with supporting beams 24 and 25 in their mostseparated positions, gripper 18 can freely pass through the bufferstation 22, even when loaded with a wafer. As shown in FIG. 2C, withsupporting beams 24 and 25 in their closest positions, a wafer can beheld on each of supporting base 26 and gripper 18 cannot pass through.

In operation, and as shown in FIG. 2D, the robot 8 arrives at integratedtool 6 loaded with a new wafer W on an arm 9. At this point, handlingsystem 16 is waiting in its uppermost position. Robot 8 places the waferW on supporting bases 26, after which, as shown in FIG. 2E, handlingsystem 16 moves down and picks up the wafer W. Robot 8 then leavesintegrated tool 6 to conduct other missions while handling system 16,loaded with the wafer W, continues down, until, as shown in FIG. 2E, itplaces the wafer, working surface down, in a measuring position on themeasuring unit 15. Typically, the measuring position includes supportswhich support the wafer on its edges (not shown). Since supporting beams24 and 25 have moved towards and away from the plane of the paper, thesupporting station 19 is shown in FIG. 2F with dashed lines.

It is noted that robot 8 leaves tool 6 empty and must arrive at tool 6unloaded in order to take back a measured wafer. Thus, robot 8 is notoptimally exploited, i.e., a disadvantage considering that the robot 8is the “bottle neck” in process environment 10 (FIG. 1).

Prior art systems solve this problem in multiple ways. One exemplaryrobot is the DBM 2400 series of Equipe Technologies, Mountain-View,Calif., USA. This robot has two separate arms. A secondary exemplaryrobot is the PerMer 6100 robot of Cybeq Systems, Sunnyvale, Calif., USA.The robot can hold two wafers, one on each side of its arm, and rotatesthe arm 180 degrees in order to switch wafers. For both prior artsystems, the robot arrives at the supporting station loaded with a newwafer, and the free arm or side faces the supporting station. The freearm (side) loads a processed wafer from the supporting station, afterwhich, the arm (side) with the new wafer is loaded onto the supportingstation. The robot then returns loaded with the processed wafer.

It will be appreciated that these solutions require additional footprintsince, during their operation, the two arms (sides) are loaded with bothnow and processed wafers. This may be a drawback in crowded processingenvironments.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to overcome theaforementioned limitations of the prior art.

There is therefore provided, in accordance with a preferred embodimentof the present invention, a buffer station for a wafer handling system.The handling system has a general path along which it moves whenhandling the wafer and the buffer station includes at least one pair ofsupporting elements and a motion unit. The supporting elements togetherare capable of supporting at least two wafers therebetween and they arelocated so as to support the wafers within the general path. The motionunit provides relative motion to the supporting elements such that, in afirst mode, the supporting elements support any of the wafers placedthereon within the general path and, in a second mode, the supportingelements are sufficiently separated so as not to disturb the motion ofthe handling system when the handling system holds a wafer. Typically,the supporting elements include at least two supporting bases eachcapable of holding a wafer thereon when the supporting elements are inthe first mode.

Alternatively, in accordance with a preferred embodiment of the presentinvention, the buffer station includes at least two pairs of supportingelements each capable of supporting at least one wafer therebetween andlocated so as to support the wafer within the general path and onemotion unit per pair of supporting elements. The motion unit shifts itsassociated pair of supporting elements in and out of the general pathand, when said supporting elements are in the general path, provides therelative motion to its associated pair of supporting elements describedhereinabove.

Additionally, each supporting element includes at least one supportingbase each capable of holding a wafer thereon when the supportingelements are in the first mode.

There is also provided, in accordance with a preferred embodiment of thepresent invention, a processing unit for processing at least onesemiconductor wafer. The unit includes a processing station forprocessing the wafer, a measuring station for measuring the wafer, arobot for moving the wafer between the processing and measuringstations, a wafer handling system and a buffer station. The waferhandling system operates in conjunction with the measuring station andmoves the wafer to and from a measuring location on the measuring unit.The buffer station is associated with the wafer handling system andreceives measured and unmeasured wafers. This enables the robot toarrive at and leave the measuring station with at least one waferthereon. The buffer station can be any of the buffer stations describedhereinabove.

Additionally, in accordance with a preferred embodiment of the presentinvention, the buffer station also includes a unit which enables therobot and the wafer handling system to operate generally independentlyof each other.

Further, in accordance with a preferred embodiment of the presentinvention, the processing unit also includes a pre-alignment unitmovably locatable within a general path of the wafer handling system.Alternatively, the pre-alignment unit can be tilted with respect to ageneral path of the wafer handling system.

Still further, the buffer station additionally operates as a centeringstation for aligning a center of the at least one wafer with a center ofthe measurement location.

Moreover, in accordance with a preferred embodiment of the presentinvention, the robot includes a unit which simultaneously carries atleast two wafers and the buffer station includes a unit which supportsat least two wafers.

Additionally, in accordance with a preferred embodiment of the presentinvention, the processing station is one of the following types ofprocessing stations: chemical-mechanical polisher, phototrack, exposuretool, etching equipment, physical vapor deposition tool and chemicalvapor deposition tool.

Moreover, in accordance with a preferred embodiment of the presentinvention, the measuring unit is an integrated tool.

Further, in accordance with a preferred embodiment of the presentinvention, the buffer station includes one buffer unit. Alternatively,the buffer station includes at least two buffer units each of which ismovable into and out of a general path of the wafer handling system.Alternatively, the buffer station includes three buffer units each ofwhich is movable into and out of a general path of the wafer handlingsystem. For the latter embodiment, the three buffer units include first,second and third buffer units are separated by a predetermined distanceand wherein the robot has two arms separated by the predetermineddistance.

Finally, there is also provided, in accordance with a preferredembodiment of the present invention, a pre-alignment unit for a wafermeasuring unit wherein the pre-alignment unit is tilted with respect toa plane of the wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description taken in conjunction with theappended drawings in which:

FIG. 1 is a schematic illustration of a prior art process environment ina semiconductor fabrication plant;

FIGS. 2A, 2B, 2C, 2D, 2E and 2F are schematic illustrations of anintegrated metrology tool and its handling system during prior artoperation with a robot;

FIG. 3A is a schematic illustration of an integrated monitoring tool andhandling system, constructed and operative in accordance with apreferred embodiment of the present invention;

FIG. 3B is a schematic illustration of a buffer station forming part ofthe handling system of FIG. 3A;

FIGS. 4A, 4B and 4C are schematic illustrations indicating the operationof the system of FIG. 3A;

FIGS. 5A and 5B are schematic illustrations indicating a centeringoperation of the system of FIG. 3A;

FIGS. 6A and 6B are two schematic illustrations of an alternativeembodiment of the integrated monitoring tool of the present inventionhaving a pre-alignment unit, in two different stages of operation;

FIGS. 6C and 6D are schematic illustrations of a wafer in various stagesof alignment, useful in understanding the operation of the pre-alignmentunit of FIGS. 6A and 6B;

FIGS. 7A and 7B are schematic illustrations of an alternative embodimentof the pre-alignment unit, in two different stages of operation;

FIG. 8 is a schematic illustration of integrating monitoring tool andhandling system having two buffer stations, constructed and operative inaccordance with an alternative preferred embodiment of the presentinvention;

FIGS. 9A, 9B and 9C are schematic illustrations indicating the operationof the system of FIG. 8;

FIG. 10 is a schematic illustration of an integrated monitoring tool andhandling system having three buffer stations, constructed and operativein accordance with a further preferred embodiment of the presentinvention;

FIGS. 11A, 11B and 11C are schematic illustrations indicating a firstoperation of the system of FIG. 10; and

FIGS. 12A and 12B are schematic illustrations indicating a secondoperation of the system of FIG. 10 with a robot having two arms.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention provides a handling system for an integratedmonitoring tool which includes a buffer station. Thus, the robotoperation becomes less dependent or even independent of the operation ofthe integrated monitoring tool, generally resulting in increasedthroughput.

The present invention can be applied to any type of integratedmonitoring tool. The term “integrated monitoring tool”, as used herein,refers to a monitoring (e.g., metrology, inspection) apparatus that ispreferably physically installed inside a processing unit or attached toit. However, it can also be separated form the processing unit, asnecessary. The monitoring tool is usually dedicated to the specificprocessing unit and wafers are preferably transferred to the apparatusby the same robot which serves the processing unit. As mentionedhereinabove, one exemplary integrated monitoring tool is the NovaScan210, but the present invention incorporates any other integratedmonitoring tool.

Furthermore, the present invention can be applied to any processingenvironment where a wafer is to be unloaded by a robot at a location(e.g., stand along metrology or inspection tool) from which the samewafer should later be reloaded by a robot, the same one or otherwise.

The processing unit to which the integrated monitoring tool is attachedcan be any processing unit in a fabrication plant. For example, it canbe a chemical mechanical polisher, such as those manufactured byStrasbaugh Inc. of San- Luis -Obispo, Calif., by Applied Materials Inc.of Santa Clara, Calif., or a phototrack manufactured by SVG SiliconValley Group of San-Jose, Calif., or by FSI International of Chaska,Minn., all of the USA. The processing unit can also be an etching,physical vapor deposition unit or chemical vapor deposition unit.

Reference is made to FIGS. 3A and 3B which respectively illustrate ahandling system 20 for the integrated tool, constructed and operative inaccordance with a preferred embodiment of the present invention, and abuffer station 22 forming part of the handling system 20. FIG. 3A is anisometric illustration of the handling system and FIG. 3B is a schematicfront view of buffer station 22. FIG. 3 also shows elements discussedpreviously and thus, similar reference numerals refer to similarelements.

The handling system 20 is similar to that described in FIG. 2 and thus,will not be described herein in more detail. Buffer station 22 issimilar to supporting station 19 and comprises two supporting beams 24and 25. However, in accordance with a preferred embodiment of thepresent invention, buffer station 22 has two supporting bases 26 and 27,detailed in FIG. 3B, rather than a single base 26 as in the prior art.This enables buffer station 22 to buffer wafers such that a standard,one-armed (i.e. single end-effector) robot can arrive with a new waferand can return with a processed wafer, rather than arriving or returningempty, as in the prior art.

Similarly to supporting station 19, supporting beams 24 and 25 areconnected to rail 30 by relative motion unit 32. Unit 32 enablessupporting beams 24 and 25 to move toward and away from each other, asindicated by arrows 34 and 36.

Reference is made to FIGS. 4A, 4B and 4C which are schematic side viewsof handling system 20 of FIG. 3 with robot 8 and illustrate theircombined operation.

As illustrated in FIG. 4A, a measured wafer W1 is present in bufferstation 22 on the lower supporting bases 27. At the same time, robot 8arrives (or is already waiting) with a wafer W2, to be measured bymeasuring unit 15, and places it onto upper supporting bases 26.

From this point in time, robot 8 and handling system 20 canindependently operate, as illustrated in FIG. 4B. Handling system 20loads new wafer W2 from upper supporting bases 26. Before the robot 8finishes loading the measured wafer W1, the handling system 20 can startcentering and/or pre-aligning the wafer W2, as discussed in more detailhereinbelow. Generally simultaneously, robot 8 loads measured wafer W1onto its arm 9 from lower supporting bases 27. At the next stage, shownin FIG. 4C, robot 8 leaves tool 6 with measured wafer W1 while handlingsystem 20 can lower new wafer W2, through the now open supporting beams24 and 25 of buffer station 22, towards and on measuring unit 15. Since,in FIG. 4C, supporting beams 24 and 26 have moved towards and away fromthe plane of the paper, the buffer station 22 is shown with dashedlines.

It will be appreciated that buffer station 22 provides the followingadvantages:

1) It enables robot 8 to unload a wafer to be measured and to load ameasured wafer, in one visit to integrated tool 6. This minimizes robotmovements and thus, saves time.

2) Since buffer station 22 is mounted on handling system 20, noadditional footprints is needed for buffer station 22. However, itshould be noted that, if no footprint limitations exist, the bufferstation of the present invention can be located out of the moving pathof the gripper. This requires that the gripper have an additionaltranslation mechanism (not shown) in order to reach the buffer station.

Applicants have realized that, in addition to buffering, buffer station22 provides the ability to center the wafers prior to placing them onmeasuring unit 15. This is illustrated in FIGS. 5A and 5B to whichreference is now made.

FIG. 5A illustrates supporting beams 24 and 25 after robot 8 has placedwafer W on supporting bases 27 but before beams 24 and 25 have reachedtheir closest positions. Typically, robot 8 does not accurately placewafer W and thus, a center O of wafer W is shifted by a distance D fromits desired measurement or processing location. Typically, robot 8places wafer W unevenly such that, between the edges of wafer W andsupporting beams 24 and 25 are typically gaps x and y, respectively,where:

D≦x+y

However, as supporting beams 24 and 25 move towards their closestpositions, shown in FIG. 5B, they push wafer W between them. Sincebuffer 22 is mounted on handling system 20 so that the centers of thecircles defined by supporting bases 26 and 27 are aligned with thecenter of the measuring position, once supporting beams 24 and 25 arriveat their final position, the center O of wafer W will be at the desiredlocation. This is shown in FIG. 5B.

In an alternative embodiment of the present invention, another type ofwafer orientation, known as pre-alignment, can be performed in thevicinity of buffer station 22, although this requires additionalequipment. Reference is now made to FIGS. 6A, and 6B which illustratethis alternative embodiment in two different states, to FIGS. 6C and 6Dwhich are useful in understanding the operation of the embodiment ofFIGS. 6A and 6B and to FIGS. 7A and 7B which provide an alternativeembodiment of the pre-alignment unit.

In this embodiment of the present invention the handling systemadditionally includes a pre-alignment unit 112 located along the path ofgripper 18. In FIGS. 6A and 6B, pre-alignment unit 112 is locatedbetween buffer station 22 and measurement unit 15. Pre-alignment unit112 is similar to that described in U.S. patent application Ser. No.09/097,298, assigned to the common assignees of the present inventionand incorporated herein by reference. Accordingly, the details ofoperation of pre-alignment unit 112 will not be described herein.

As discussed in U.S. patent application Ser. No. 09/097,298, prealignment unit 112 detects the presence of a marker which is standardlypresent on wafers. Such a marker can be a flat line (a “flat”) crossinga small portion of the edges of the wafer or a notch and is used todefine the fiducial axis of the wafer.

Pre-alignment unit 112 is a moveable opto-couple detector, focused onthe edge of wafer W, which comprises a point illuminator 116, such as alight emitting diode (LED), a single photodiode 118, a photodiode lens119 and a translation mechanism, indicated by arrow 121. The translationmechanism holds pre-alignment 112 and moves it into and out of its placeabove the wafer edge.

FIG. 6A shows pre-alignment unit 112 in a first, detecting position,surrounding an edge area 60 of the wafer and FIG. 6B shows pre-alignmentunit 112 in a second, non-detecting position away from the path ofgripper 18.

While buffer station 22 is in its open position, gripper 18 holds waferW at the height of pre-alignment unit 112. Pre-alignment unit 112 isthen brought into the first position shown in FIG. 6A. FIG. 6Cillustrates the original arbitrary orientation of the wafer W. Asindicated by arrow 120 in FIG. 6C, gripper 18 rotates the wafer W untila marker 100 (e.g., a flat or a notch) passes the pre-alignment unit 112which then indicates such to the integrated monitoring tool's controlunit (not shown).

Specifically, the point illuminator 116 illuminates the bottom side ofthe edge area 60 of the wafer W whereas the single photodiode 118detects signals above the edge area 60. Whenever the marker is notlocated between the elements of the pre-alignment unit 112, no lightfrom the point illuminator 116, above a predetermined threshold level,can reach the photodiode 116. However, once the photodiode 118 detects asignificant signal, i.e., the marker is between the elements of thedetector 112, the control unit stops the rotation of gripper 18. Thewafer W is now in a generally known position, near the detector 112 asshown in FIG. 6D, although its precise orientation is still unknown.

The pre-alignment unit 112 is now returned to the side, as shown in FIG.6B, and, typically, gripper 18 brings the now pre-aligned wafer W tomeasurement unit 15.

FIGS. 7A and 7B illustrate another preferred embodiment of thepre-alignment unit 112 which eliminates the need to insert and removethe pre-alignment unit 112 to and from the gripper's 18 translationpath. FIG. 7A shows a configuration in which buffer station 22 islocated between measuring unit 15 and pre-alignment unit 112.Pre-alignment unit 112 is tilted and located in a position above theuppermost position of gripper 18. This position is the furthest positionfrom measuring unit 15.

Gripper 18 first takes the wafer W (generally, an unmeasured wafer) fromupper supporting bases 26 and brings it into pre-alignment unit 112.Since pre-alignment unit 112 is tilted such that its lower half is awayfrom the path of gripper 18, wafer W does not hit anything during thisoperation.

When gripper 18 is in its uppermost position, point illuminator 118illuminates the bottom side of the edge area 60 of the wafer W whilephotodiode 118 detects signals above the edge area 60. Whenpre-alignment is completed, gripper 18 lowers the wafer, away frompre-alignment unit 112, through the now open buffer station 22, tomeasuring unit 15, as shown in FIG. 7B. Once again, since pre-alignmentunit 112 is tilted, gripper 18 can move wafer W into and out of unit 112without unit 112 having to move.

It is noted that the buffer station 22 can be used with an integratedmonitoring tool in which the measuring unit is located above thehandling system. However, for this alternative embodiment, supportingmeans 24 and 25 should be placed so that their supporting bases 26 and27 face the measuring unit rather than as in the previous embodiment.Pre-alignment unit 112 would then be below buffer station 22, in thelocation furthest from measuring unit 15.

Reference is made to FIG. 8 which illustrates a handling system 60according to another preferred embodiment of the present inventionhaving a buffer station with two pairs of supporting arms 62A and 62B.Similar reference numbers refer to similar elements. Reference is alsomade to FIGS. 9A, 9B and 9C which illustrate the combined operation ofhandling system 60, buffer stations 62A and 62B and the robot 8.

As in the previous embodiment, pairs of supporting arms 62A and 62B canbe located above measuring unit 15 and, typically, they have elementssimilar to those of buffer station 22. However, in this embodiment, eachpair of supporting arms 62A and 62B is separately movable away from thepath of gripper 18, as indicated by arrow 63, typically via a side rail64 which is controlled by a motor (not shown). Thus, FIG. 8 shows arms62A within the path of gripper 18 while arms 62B are out of the path. Asin the previous embodiment, each pair of supporting arms 62A and 62Bincludes relative motion mechanism 32 which separates supporting beams24 and 25 enough to allow the passage of gripper 18 therethrough.

It is noted that, in this embodiment, each side rail 64 is mounted onsolid connector 74 and has a side translation unit 63 along side rail 64via side translation unit 63. However, it should be emphasized that themovement of a pair of supporting arms into and out of the path ofgripper 18 (and to any intermediate point as well) can be realized byany other suitable non linear motion, e.g., rotation.

According to a preferred embodiment, it is sufficient that eachsupporting arm have a single supporting base 26. However, FIG. 8 showseach supporting arm 24 and 26 with two supporting bases 26 and 27 whichincreases the buffering capacity of the buffer station, as will bedescribed hereinbelow.

FIGS. 9A, 9B and 9C illustrate the operation of handling system 60. Asillustrated by FIG. 9A, pair of supporting arms 62A is in the openposition within the path of gripper 18 while pair of supporting arms 62Bis in the closed position out of the path of gripper 18.

A measured wafer W1 is present on lower supporting bases 27B of pair ofsupporting arms 62B while a second wafer W2 is handled by gripper 18e.g., is being measured or is being placed in a measuring position onmeasuring unit 15. Generally simultaneously, robot 8 arrives with athird wafer W3 to be measured, and places it on the uppermost supportingbases 26B of pair of supporting arms 62B.

As illustrated by FIG. 9B, when robot 8 finishes placing new wafer W3 onupper supporting bases 26B, robot 8 takes measured wafer W1 from lowersupporting bases 27B and returns to conduct other missions. Generallysimultaneously, pair of supporting arms 62A closes and gripper 18 placesnow measured second wafer W2 on supporting bases 27A.

As illustrated by FIG. 9C, pairs of supporting arms 62A and 62B changepositions, such that pair of supporting arms 62B, with new wafer W3thereon, enters the path of gripper 18 while pair of supporting arms62A, with measured wafer W2 thereon, moves out of the path of gripper18. Gripper 18 can now load third wafer W3 from upper supporting bases26B and, once pair of supporting arms 62B opens up, can place new waferW3 on measuring unit 15. When the measurement of the third wafer W3 isfinished, it will be placed by gripper 18 onto supporting bases 27B andthe buffering cycle continues. In the meantime, robot 8 can loadmeasured wafer W2.

Thus, this buffering method enables robot 8 to unload a new wafer to bemeasured and to load a measured wafer, generally, while a third wafer isbeing handled by handling system 20. Thus, robot 8 and handling system20 are relatively independent of each other in this embodiment.

Reference is made to FIGS. 10 and 11 which illustrate a further handlingsystem 70 and its operation, respectively. In this embodiment handlingsystem 70 comprises three pairs of supporting arms 72A, 72B and 72C,each formed in a manner similar to pairs of supporting arms 62 of theprevious embodiment. Similar reference numerals refer to similarelements. Thus, each pair of supporting arms 72 has supporting beams 24and 25 and each pair of supporting arms 72 moves between an in-path andan out-of-path position with respect to gripper 18. Typically, only onepair of supporting arms 72 is in the in-path position at any given time.

FIGS. 11A, 11B and 11C show the operation of handing system 70. Thisembodiment is particularly useful for a process environment with highthroughput and thus, a high buffer capacity is needed to enable theoperation of robot 8 and tool 6 to be sufficiently independent of eachother.

FIG. 11A illustrates a point in time at which two measured wafers W1 andW2 are already present on the lower supporting bases 27B and 27C ofout-of-path pairs of supporting arms 72B and 72C, respectively. At thesame time, another wafer W4 is being handled by gripper 18 (e.g., it isbeing measured or it is being brought to or from measuring unit 15).Accordingly, pair of supporting arms 72A is open in the in-pathposition. FIG. 11A also shows robot 8 loading a new wafer W3 onto upperbases 26B of pair of supporting arms 72B.

As illustrated in FIG. 11B, when robot 8 finishes unloading and placingnew wafer W3, it takes measured wafer W1 from lower supporting bases 27Bof pair of supporting arms 72B and returns to its other missions.According to this preferred embodiment, robot 8 can generallyimmediately return back with a new wafer W5, as illustrated in FIG. 11C,and can place it on any available supporting base, such as base 27C.

While robot 8 is performing its operations, gripper 18 moves wafer W4through the measurement process. Thus, it is shown on measuring unit 16in FIG. 11B and on supporting bases 27A of closed pair of supportingarms 72A in FIG. 11C.

It is noted that, in general, pairs of supporting arms can be added toincrease the buffer capacity of the handling system. It will beappreciated that, although two supporting bases are shown for each pairof supporting arms station, it is possible to have more or less, asneeded.

It will be appreciated that the present invention can operate with arobot 80 which can simultaneously carry two or more wafers (e.g., a semior full wafer cassette). This is illustrated in FIGS. 12A and 12B towhich reference is now made. The embodiment of FIGS. 12A and 12B useshandling system 70 of FIG. 10 and thus, similar reference numerals referto similar elements.

In this embodiment, robot 80 must be able to simultaneously deliver allits wafers to the relevant buffer station, e.g., each wafer should beplaced onto a different supporting base on different supporting arms.Therefore, as illustrated in FIG. 12A, two measured wafers W1 and W2 arepresent on the lower supporting bases 27B and 27C, respectively, whilepairs of supporting arms 72B and 72C are in the out-of-path position. Atthe same time, another wafer W3 is handled by gripper 18 while pair ofsupporting arms 72A is in the in-path position. Generallysimultaneously, robot 80 carries two new wafers W4 and W5 on its twoarms 9A and 9B, respectively.

It is noted that a height difference d1 between upper supporting bases27B and 27C of pairs of supporting arms 72B and 72C is generally equalto a height difference d2 between the wafers W4 and W5 carried on arm 9Aand 9B of robot 80. When robot 80 arrives at pairs of supporting arms72B and 72C, it can unload wafers W4 and W5 together to the uppersupporting bases 26B and 26C and then it can load measured wafers W1 andW2 together from lower supporting bases 27B and 27C, after which itreturns to the other missions, as illustrated in FIG. 12B.

It will be appreciated that each of the embodiments shown herein canalso include pre-alignment unit 112 (FIGS. 6 and 7) and can perform thecentering operations shown in FIGS. 5A and 5B.

It will be appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed herein above. Rather the scope of the invention is defined bythe claims that follow:

What is claimed is:
 1. A buffer station for a wafer handling system, thehandling system having a general path along which it moves when handlingthe wafer, the buffer station comprising: at least one pair ofsupporting elements capable of supporting at least two waferstherebetween and located so as to support said wafers within saidgeneral path; and motion means for providing relative motion to saidsupporting elements such that, in a first mode, said supporting elementssupport any of said wafers placed thereon within said general path and,in a second mode, said supporting elements are sufficiently separated soas not to disturb the motion of said handling system when said handlingsystem holds a wafer.
 2. A buffer station according to claim 1 andwherein said supporting elements include at least two supporting baseseach capable of holding a wafer thereon when said supporting elementsare in said first mode.
 3. A buffer station according to claim 2 andwherein each of said supporting elements include at least one supportingbase each capable of holding a wafer thereon when said supportingelements are in said first mode.
 4. A buffer station according to claim3 and wherein said at least one supporting base including two supportingbases.
 5. A buffer station for a wafer handling system, the handlingsystem having a general path along which it moves when handling thewafer, the buffer station comprising: at least two pairs of supportingelements each capable of supporting at least one wafer therebetween andlocated so as to support said wafer within said general path; and onemotion means per pair of supporting elements shifting its associatedpair of supporting elements in and out of said general path and whensaid supporting elements are in said general path, for providingrelative motion to its associated pair of supporting elements such that,in a first mode, said supporting elements support said wafer within saidgeneral path and, in a second mode, said supporting elements aresufficiently separated so as not to disturb the motion of said handlingsystem when said handling system holds a wafer.
 6. A buffer stationaccording to claim 5 and wherein said at least two pairs of supportingelements includes three pairs of supporting elements.